I use this pattern to test for undefined and null values in ActionScript/Flex :
if(obj) {
execute()
}
Unfortunately, a ReferenceError is always thrown when I use the pattern to test for child objects :
if(obj.child) {
execute()
}
ReferenceError: Error #1069: Property child not found on obj and there is no default value.
Why does testing for child objects with if statements throw a ReferenceError?
Thanks!
You're getting this error because the obj's type does not have the child property in it. You need to do something like this:
if((obj) && (obj.hasOwnProperty('child') && (obj.child)){
execute()
}
More info on the hasOwnProperty method in the Object class:
http://livedocs.adobe.com/flex/3/langref/Object.html#hasOwnProperty%28%29
This happens when obj is a strongly typed object but it doesn't have a child field.
You can test to see if a field exists on any object using the in operator:
if ("foo" in obj && obj.foo)
execute();
I've also written a utility function to make this process easier:
function getattr(obj:Object, field:*, dflt:*=undefined):* {
if (field in obj && obj[field])
return obj[field];
return dflt;
}
You can avoid reference errors by using array notation:
if([obj.name][child.name]){
execute();
}
The important thing to realize is that simply avoiding the error can cause issues down the track - debugging will be harder in larger applications.
Of course, the ideal approach is to completely avoid the situation by using validator functions to ensure you have the right data, instead of testing for null when the data is required. :)
Related
I've been trying some stuff from kotlin.reflection during my project, and got stuck on something what occurs to me as hard to understand, I have declared object as follows:
object WebsiteMapping
{
const val ADMIN = "/admin"
}
once I call:
Arrays
.stream(WebsiteMapping::class.java.declaredFields)
.forEach { field -> println(field.type) }
what I get is:
class java.lang.String
class mapping.WebsiteMapping
When I looked a little bit into what is behind declaredFields invocation I grasped why it works as it is, but is there any convenient way of taking only declared consts within that object without getting also root of the whole structure?
The field with the type class mapping.WebsiteMapping is, basically, not the root of the structure but a special field generated in the object type that holds the reference to the singleton object.
In Kotlin, this field is named INSTANCE by convention. You can therefore filter the fields that you get from the class as follows:
WebsiteMapping::class.java.declaredFields
.filter { it.name != "INSTANCE" }
.forEach { println(it.type) }
Another solution is to switch from java.reflect.* to the Kotlin reflection API kotlin.reflect (needs a dependency on the kotlin-reflect module), which automatically filters the property:
WebsiteMapping::class.memberProperties
.forEach { println(it.returnType) }
I want to iterate over all fields in one of my classes, filter for annotated ones and then check if the field has one specific type.
All I found was field.returnType.isSubtype(other: KType) but I don't know how to get the KType of my other class.
Here is my code so far:
target.declaredMemberProperties.forEach {
if (it.findAnnotation<FromOwner>() != null) {
if ( /* it.returnType is Component <- Here I need some working check */ ) {
// do stuff
} else {
// do ther stuff
}
}
}
There are at least two solutions here:
Get the KClass<*> of it.returnType using .jvmErasure, then check the subtype relationship for the KClasses:
it.returnType.jvmErasure.isSubclassOf(Component::class)
Since Kotlin 1.1, you can construct the KType from the KClass token using .createType() (check its optional parameters: you can use them to provide nullability info, type arguments and annotations), and then check the subtype as you suggested:
it.returnType.isSubtypeOf(Component::class.createType())
Creating the type on every iteration may introduce performance issues, make sure you cache it if you need it often.
In flex, how to check if a variable exists? I have tried using
if (this['some_variable'] != undefined) {
//do something
}
There is a run time error saying the property some_variable does not exists. I have checked with null instead of undefined, still the same error.
please help.
[EDIT]
Based on the replies I have used this.hasOwnProperty('variable_name'). I found that its returning true if variable_name is a public but false if its private/protected. How to check for a private variable?
There are two ways for that:
if ("some_variable" in this) {
//do something
}
It uses in operator.
And:
if (this.hasOwnProperty("some_variable")) {
//do something
}
See documentation about hasOwnProperty().
What about getting information about private/protected properties the situation is that you can't get this info with the current state of Flash Player. The only possible way, I suppose, is some kind of runtime bytecode manipulation. But as far as I know nobody implemented it yet.
But I have a question about getting info about private/protected properties: for what purpose you need it? The nature of these properties/methods is you can't call them. Even if you know about their existence.
You can use
if (this. hasOwnProperty("some_variable")) {
//access the variable inside
}
if (this.hasOwnProperty('some_variable')) DO_IT_!()
Explanation:
this['some_variable'] tries to evaluate the value of the instance property some_variable. If there is no such a property, you will get this error.
To test if a property exists for a particular object use hasOwnProperty or wrap your condition in a try/catch block or use if ('some_variable' in this).
Usually you create an object property in a class file:
public class MyClass {
public var myProperty : String = "ich bin hier";
}
Then you refer to that property within the class:
trace (myProperty);
trace (this.myProperty);
Using the array syntax [] is also possible but will throw the error if the property is not defined.
trace (this['myProperty']);
And finally! If you declare your class to be dynamic you might use the array syntax even if the property does not exist.
public dynamic class MyClass {
public function MyClass() {
trace (this["never_declared_property"]);
}
}
So I have a class where I instantiate a variable callback like so:
public var callback:Function;
So far so good. Now, I want to add an event listener to this class and test for existence of the callback. I'm doing like so:
this.addEventListener(MouseEvent.MOUSE_OVER, function(event:MouseEvent) : void {
if (callback) {
// do some things
}
});
This works great, doesn't throw any errors, but everywhere I test for callback I get the following warning:
3553: Function value used where type Boolean was expected.
Possibly the parentheses () are missing after this function reference.
That bugged me, so I tried to get rid of the warning by testing for null and undefined. Those caused errors. I can't instantiate a Function as null, either.
I know, I know, real programmers only care about errors, not warnings. I will survive if this situation is not resolved. But it bothers me! :) Am I just being neurotic, or is there actually some way to test whether a real Function has been created without the IDE bitching about it?
Similar to using typeof:
if(callback is Function){
}
I believe should evaluate to true if the function exists and is a function and false if it is null or is not a function. (although if that doesn't work try if(callback && callback is function){}
if( !(callback == null)){
// do something
}
There's already an answer that works, but I thought I'd mention that you can also stop the warning from occurring by explicitly casting the result to a Boolean.
if (Boolean(callback)) {
// do something
}
Have you tried:
if (typeof callback == "function") {
// do some things
}
?
http://www.adobe.com/livedocs/flash/9.0/ActionScriptLangRefV3/operators.html#typeof
I am a Java programmer and need to work on a Flex/ActionScript project right now. I got an example of using ITreeDataDesriptor from Flex 3 Cookbook, but there is one line of actionscript code that's hard for me to understand. I appreciate if someone could explain this a little further.
public function getData(node:Object, model:Object=null):Object
{
if (node is Office) {
return {children:{label:node.name, label:node.address}};
}
}
The part that I didn't understand was "{children:{label:node.name, label:node.address}}". Office is simply a value object that contains two String properties: name and address.
The following return expression (modified from the question) ...
return {children:{label:node.name, body:node.address}}
... is functionally equivalent to this code ...
var obj:Object = new Object();
obj.children = new Object();
obj.children.label = node.name;
obj.children.body = node.address;
return obj;
The anonymous object returned in the question code complicates matters because it defines a property twice. In that case, the first declaration is used, and the subsequent one(s) are ignored. No compile-time or runtime error is thrown.
I think in Java you would call that a map or an associative array. In Javascript and Actionscript you can say this to create an object with certain properties:
var myobject = {
'prop1': 100,
'prop2': {
'a': 1
}
}
trace( myobject.prop1 ); // 100
trace( myobject.prop2.a ); // 1
In your example it's just returned as a nameless object.
return {children:{label:node.name, label:node.address}};
Means you are returning a new Object. The {} are the Object's constructor, and in this case its an Anonymous object.
Thank you both for the quick response. So if I understand your explanations correctly, the return statement is returning an anonymous object, and this object has only one property named "children", which is again an associative array - ok, here is the part I don't quite understand still, it seems that both properties in this array are named "label", is this allowed?